Caustic scrubber system and method for biogas treatment

ABSTRACT

The present invention provides a method and apparatus for the treatment of process gas from an anaerobic digestion system or a landfill gas system. In one embodiment, the system comprises a caustic scrubber including a vertical column having a top and a bottom and including a counter current flow system, wherein a process gas stream flows up vertically through the column in counter current flow to a caustic liquid solution that flows downward through the column. The caustic liquid solution removes at least one acid from the process gas stream, wherein treated gas that is substantially free of acids bubbles out through an opening at the top of the vertical column.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application No.13/276,199, filed Oct. 18, 2011, the content of which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

The invention broadly relates to biogas applications and, moreparticularly, to a caustic scrubber system and method for biogastreatment.

BACKGROUND OF THE INVENTION

Biogas refers to a gaseous fuel produced by the biological breakdown oforganic matter in the absence of oxygen. It is produced by the anaerobicdigestion or fermentation of biodegradable materials such as biomass,manure, sewage, municipal waste, green waste, plant material and crops.Biogas primarily comprises methane and carbon dioxide, and may containsmall amounts of hydrogen sulphide, moisture and siloxanes.

The gases methane, hydrogen and carbon monoxide can be combusted oroxidized with oxygen. This energy release allows biogas to be used as afuel. Biogas can be used as a fuel for any heating purpose. It can alsobe produced by anaerobic digesters where it is typically used in a gasengine to convert the chemical energy of the gas into electricity andheat. Anaerobic digestion is a series of processes in whichmicroorganisms break down biodegradable material in the absence ofoxygen, also used for industrial or domestic purposes to manage wasteand/or to release energy.

The digestion process begins with bacterial hydrolysis of the inputmaterials in order to break down insoluble organic polymers such ascarbohydrates and make them available for other bacteria. Acidogenicbacteria then convert the sugars and amino acids into carbon dioxide,hydrogen, ammonia, and organic acids. These bacteria then convert theseresulting organic acids into acetic acid, along with additional ammonia,hydrogen, and carbon dioxide. Finally, methanogens convert theseproducts to methane and carbon dioxide.

Anaerobic digesters can use a multitude of feed stocks for theproduction of methane rich bio-gas including but not limited topurpose-grown energy crops such as maize. Landfills also produce methanerich bio-gas through the anaerobic digestion process. As part of anintegrated waste management system, this bio-gas may be collected andprocessed for beneficial use while simultaneously reducing greenhousegas emissions into the atmosphere.

Anaerobic digestion is widely used as a source of renewable energy. Theprocess produces a biogas that can be used directly as cooking fuel, incombined heat and power gas engines or upgraded to natural gas qualitybiomethane. The utilization of biogas as a fuel helps to replace fossilfuels. The nutrient-rich digestate and/or Leachate that is also producedcan be used as fertilizer.

The technical expertise required to maintain industrial scale anaerobicdigesters coupled with high capital costs and low process efficiencieshave limited the level of its industrial application as a wastetreatment technology. As a result, it is imperative that anaerobicdigesters and landfill gas treatment plants operate at the highestpossible efficiency.

In biogas applications such as landfills and digesters, H₂S and otherimpurities including halides and halogenated compounds are frequentlypresent in low percent to ppm/ppb quantities. These compounds maydissociate at high temperatures and in the presence of water to formcaustic acids including, but not limited to H₂S, HF, H₂SO₄, H₃PO₄ andHNO₃.

Typical metallurgy such as carbon and stainless steels are susceptibleto corrosion and failure when placed into contact with these acids.Downstream equipment that changes the dew point and allows condensationto occur may concentrate these acids in pooling areas such as moistureseparators, chillers, and gas coolers.

Gas processing techniques and other unit operations may produce acidsform gas constituents. However, these systems merely employ acidneutralization after the acids have formed and concentrated in thepooling areas. As such, these conventional systems simply act as a bandaid to condensation. Caustic scrubbers have been used in the past forseveral applications. For example, they may be used for CO₂ removal, H₂Sremoval and also for the removal of several other reactive contaminantsin both liquid and gaseous phase.

SUMMARY OF THE INVENTION

The present invention provides a caustic scrubber system and method forbiogas treatment. The caustic scrubber may be used to neutralize acidssuch as H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃ in either the liquid or gaseousphase to protect downstream metallurgy and equipment.

One embodiment of the invention is directed toward an apparatus for thetreatment of process gas from an anaerobic digestion system or alandfill gas system, the apparatus comprising a caustic scrubberincluding a vertical column having a top and a bottom and including acounter current flow system, wherein a process gas stream flows upvertically through the column in counter current flow to a causticliquid solution that flows downward through the column. The causticliquid solution removes at least one acid from the process gas stream,wherein treated gas that is substantially free of acids bubbles outthrough an opening at the top of the vertical column.

In the above-described apparatus, the at least one acid can be selectedfrom the group consisting of H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃. Inaddition, the liquid solution may comprise a basic solution forneutralizing the at least one acid. In one embodiment, the causticliquid solution comprises an NaOH solution. The caustic liquid solutionmay be gravity trickled or sprayed within the vertical column. The spentcaustic liquid solution containing the at least one acid may flowthrough an opening in the bottom of the column and into a neutralizationtank.

According to some embodiments, the treated gas flows back into theanaerobic digestion system. In one implementation, the caustic scrubberis located within the anaerobic digestion system between an anaerobicdigester and a biogas handling system in order to protect the biogashandling system and other downstream equipment from corrosion. Thecaustic scrubber allows for the total removal of H₂S, HF, H₂SO₄, H₃PO₄and HNO₃ from the process gas stream so that acid concentration does notoccur in any downstream areas regardless of changing dew point andcondensation.

Another embodiment of the invention is directed toward an apparatus forthe treatment of process gas from a landfill gas system, comprising acaustic scrubber including a vertical column having a top and a bottomand including a counter current flow system, wherein a process gasstream flows up vertically through the column in counter current flow toa caustic liquid solution that flows downward through the column;wherein the caustic liquid solution removes at least one acid from theprocess gas stream; and wherein treated gas that is substantially freeof acids bubbles out through an opening at the top of the verticalcolumn.

Yet another embodiment of the invention comprises a method of using acaustic scrubber for the treatment of process gas from an anaerobicdigestion system. In particular, the method may entail: (i) receiving aprocess gas stream from the anaerobic digestion system; (ii) flowing theprocess gas stream up vertically through a vertical column; (iii)flowing a caustic liquid solution downward through the vertical columnsuch that the caustic liquid solution removes at least one acid from theprocess gas stream; (iv) bubbling out treated gas that is substantiallyfree of acids through an opening at the top of the vertical column; and(v) flowing the treated gas into an anaerobic digestion cleanup system.

A further embodiment of the invention comprises a method of using acaustic scrubber for the treatment of process gas from a landfill gassystem. Specifically, the method may entail: (i) receiving a process gasstream from the landfill gas system; (ii) flowing the process gas streamup vertically through a vertical column; (iii) flowing a caustic liquidsolution downward through the vertical column such that the causticliquid solution removes at least one acid from the process gas stream;(iv) bubbling out treated gas that is substantially free of acids anopening at the top of the vertical column; and (v) flowing the treatedgas into a landfill gas cleanup system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating the stages of an exemplaryanaerobic digestion system.

FIG. 2 is a diagram illustrating the stages of an exemplary landfill gassystem.

FIG. 3 is a illustrating the use of a caustic scrubber system inaccordance with an embodiment of the invention.

DETAILED DESCRIPTION

In the following paragraphs, the present invention will be described indetail by way of example with reference to the attached drawings.Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars, rather than as limitations on thepresent invention. As used herein, the “present invention” refers to anyone of the embodiments of the invention described herein, and anyequivalents. Furthermore, reference to various feature(s) of the“present invention” throughout this document does not mean that allclaimed embodiments or methods must include the referenced feature(s).

Biogas is a renewable energy composed primarily of methane resultingfrom the natural decomposition of organic waste by anaerobic bacteria.Similar to natural gas, methane captured by a biogas system can be usedto provide heat, electrical power or transportation biofuel. Biogasextraction can be used to: (i) produce green and renewable energy; (ii)reduce pollution and greenhouse gases; (iii) reduce waste odors andpathogens; and transform waste into valuable bio-fertilizer.

Fermentation, or anaerobic digestion, is the most common process thatbreaks down the organic waste. The organic waste may then be oxidized,thereby creating energy. Various types of organic materials include, butare not limited to: (i) biomass, (ii) landfill waste, (iii) sewage, (iv)manure, and (v) plant material. The most common gases produced aremethane and carbon dioxide. Other gases that can be formed includehydrogen, nitrogen, and carbon monoxide. Methane, hydrogen, and carbonmonoxide can be combusted to create heat and electricity. When biogas iscreated from existing waste streams, it reduces odors and methaneemissions and creates two renewable resources. Methane is a potentgreenhouse gas that contributes to global climate change. It is expectedthat a landfill gas energy project will capture about 60% to 90% of themethane emitted from the landfill, depending on system design andeffectiveness.

There are two primary methods of recovering biogas for use as energy,namely: (i) by creating an anaerobic digestion system to process waste,most commonly manure or other wet biomass, and (ii) by recoveringnatural biogas production formed in existing landfills. Once recovered,biogas can be converted to energy using a number of methods.

FIG. 1 is a flow diagram illustrating the stages of an exemplaryanaerobic digestion system 100. Specifically, the an anaerobic digestionsystem 100 comprises a manure collection system 110, a manure handlingsystem 120, an anaerobic digester 130, a biogas handling system 140, gasuse devices 150, an effluent storage 160. In addition, at least oneflare 170 may be used to burn excess gas. Digester products 180 may beused for bedding, potting soil, land applications, etc. Moreparticularly, manure collection system 110 is used to gather manure andtransport it to the anaerobic digester 130. In some cases, existingliquid/slurry manure management systems can be adapted to deliver manureto the anaerobic digester 130. The anaerobic digester 130 may bedesigned to stabilize manure and optimize the production of methane. Astorage facility for digester effluent, or waste matter, is alsorequired.

With further reference to FIG. 1, the anaerobic digester 130 outputsbiogas into the biogas handling system 140. The biogas may containapproximately 60% methane and 40% carbon dioxide. It is collected,treated, and piped to a gas use device 150. By way of example, thebiogas can then be upgraded to natural gas pipeline quality. It may alsobe used to generate electricity, as a boiler fuel for space or waterheating, or for a variety of other uses. At least one flare 170 is alsoinstalled to destroy extra gas and as a back-up mechanism for theprimary gas use device 160.

The anaerobic digester 130 may be made out of concrete, steel, brick, orplastic. Additionally, the digester 130 includes a tank for pre-mixingthe waste and a digester vessel. In some embodiments, the anaerobicdigester 130 may comprise a batch digesters or a continuous digester. Abatch digester is loaded with organic materials, which are allowed todigest therein. The retention time depends on temperature and otherfactors. Once the digestion is complete, the effluent is removed and theprocess is repeated.

In further embodiments, the anaerobic digester 130 may comprise acontinuous digester, wherein organic material is constantly or regularlyfed into the digester, and wherein the material moves through thedigester either mechanically or by the force of the new feed. Unlikebatch-type digesters, continuous digesters produce biogas without theinterruption of loading material and unloading effluent. Various typesof continuous digesters include vertical tank systems, horizontal tankor plug-flow systems, and multiple tank systems.

Anaerobic digestion also occurs naturally underground in landfills,wherein the waste is covered and compressed by the weight of thematerial that is deposited above. This material prevents oxygenexposure, thereby allowing chemical reactions and microbes to act uponthe waste. This encourages an uncontrolled process of biomass decay. Therate of production is affected by waste composition and landfillgeometry. Landfill gas may comprise about 40% to 60% methane, and about40% to 60% carbon dioxide.

FIG. 2 is a diagram illustrating an exemplary landfill gas system 200including landfill 210, landfill gas wells 220 for active gascollection, landfill gas wellhead 230, landfill gas processing andtreatment plant 240, and at least one landfill gas flare 250. Landfillgas is extracted from landfill 210 using a series of wells 220 and ablower/flare system. The landfill gas system 200 directs the collectedgas to landfill gas processing and treatment plant 240, where it isprocessed and treated.

FIG. 3 is a diagram illustrating the use of a caustic scrubber system300 in accordance with an embodiment of the invention. In particular,process gas 310 (e.g., from an anaerobic digestion system 100 or alandfill gas system 200) is fed into the caustic scrubber 320. Processgas 310 contains at least one acid selected from the group consisting ofH₂S, HF, H₂SO₄, H₃PO₄ and HNO₃. Such acids may be present in liquidand/or gaseous phases. According to various embodiments, the causticscrubber 320 comprises a vertical column and includes a counter currentflow system, wherein the gas stream travels up vertically through thecolumn (as indicated by arrow 330) in counter current flow to a gravitytrickling or sprayed caustic liquid solution that flows downward withinthe column (as indicated by arrow 340). The caustic liquid solutioncomprises a basic solution that is employed to neutralize various acidsthat are likely to be present in the process gas 310 such as H₂S, HF,H₂SO₄, H₃PO₄ and HNO₃. By way of example, the caustic liquid solutionmay comprise an NaOH solution. This process results in treated gas 350that is substantially free of acids bubbling out through an opening atthe top of the caustic scrubber 320. The spent caustic liquid solutioncontaining acids flows through an opening in the bottom of the columnand into a neutralization tank 360.

With further reference to FIG. 3, the treated gas 350 that issubstantially free of acids flows back into the anaerobic digestionsystem 100 (FIG. 1) or the landfill gas system 200 (FIG. 2). In order toprotect downstream metallurgy and equipment, a caustic scrubber 320 maybe located, for example, between the anaerobic digester 130 and thebiogas handling system 140 (FIG. 1) in order to protect the biogashandling system 140 and other downstream equipment from corrosion andother deleterious effects. Additionally, a caustic scrubber 320 may belocated between the landfill gas wells 220 and the landfill gasprocessing and treatment plant 240 (FIG. 2) in order to protect thelandfill gas processing and treatment plant 240 and other downstreamequipment from corrosion. According to some embodiments of theinvention, the caustic scrubber allows for the total removal of acidssuch as H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃ from the gas stream so that acidconcentration cannot occur in any downstream areas regardless ofchanging dew point and condensation.

Another embodiment of the invention comprises a method of using acaustic scrubber for the treatment of process gas from an anaerobicdigestion system. In particular, the method may entail: (i) receiving aprocess gas stream from the anaerobic digestion system; (ii) flowing theprocess gas stream up vertically through a vertical column; (iii)flowing a caustic liquid solution downward through the vertical columnsuch that the caustic liquid solution removes at least one acid from theprocess gas stream; (iv) bubbling out treated gas that is substantiallyfree of acids an opening at the top of the vertical column; and (v)flowing the treated gas into an anaerobic digestion cleanup system.

A further embodiment of the invention comprises a method of using acaustic scrubber for the treatment of process gas from a landfill gassystem. Specifically, the method may entail: (i) receiving a process gasstream from the landfill gas system; (ii) flowing the process gas streamup vertically through a vertical column; (iii) flowing a caustic liquidsolution downward through the vertical column such that the causticliquid solution removes at least one acid from the process gas stream;(iv) bubbling out treated gas that is substantially free of acids anopening at the top of the vertical column; and (v) flowing the treatedgas into a landfill gas cleanup system.

One skilled in the art will appreciate that the present invention can bepracticed by other than the various embodiments and preferredembodiments, which are presented in this description for purposes ofillustration and not of limitation, and the present invention is limitedonly by the claims that follow. It is noted that equivalents for theparticular embodiments discussed in this description may practice theinvention as well.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not of limitation. Likewise, the various diagrams maydepict an example architectural or other configuration for theinvention, which is done to aid in understanding the features andfunctionality that may be included in the invention. The invention isnot restricted to the illustrated example architectures orconfigurations, but the desired features may be implemented using avariety of alternative architectures and configurations. Indeed, it willbe apparent to one of skill in the art how alternative functional,logical or physical partitioning and configurations may be implementedto implement the desired features of the present invention. Also, amultitude of different constituent module names other than thosedepicted herein may be applied to the various partitions. Additionally,with regard to flow diagrams, operational descriptions and methodclaims, the order in which the steps are presented herein shall notmandate that various embodiments be implemented to perform the recitedfunctionality in the same order unless the context dictates otherwise.

Although the invention is described above in terms of various exemplaryembodiments and implementations, it should be understood that thevarious features, aspects and functionality described in one or more ofthe individual embodiments are not limited in their applicability to theparticular embodiment with which they are described, but instead may beapplied, alone or in various combinations, to one or more of the otherembodiments of the invention, whether or not such embodiments aredescribed and whether or not such features are presented as being a partof a described embodiment. Thus the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

A group of items linked with the conjunction “and” should not be read asrequiring that each and every one of those items be present in thegrouping, but rather should be read as “and/or” unless expressly statedotherwise. Similarly, a group of items linked with the conjunction “or”should not be read as requiring mutual exclusivity among that group, butrather should also be read as “and/or” unless expressly statedotherwise. Furthermore, although items, elements or components of theinvention may be described or claimed in the singular, the plural iscontemplated to be within the scope thereof unless limitation to thesingular is explicitly stated.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, may be combined in asingle package or separately maintained and may further be distributedacross multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives may be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

The invention claimed is:
 1. A method of using a caustic scrubber forthe treatment of process gas from an anaerobic digestion system,comprising: receiving a process gas stream from the anaerobic digestionsystem; flowing the process gas stream up vertically through a verticalcolumn; flowing a caustic liquid solution downward through the verticalcolumn such that the caustic liquid solution removes at least one acidfrom the process gas stream; bubbling out treated gas that issubstantially free of acids through an opening at the top of thevertical column; and flowing the treated gas into an anaerobic digestioncleanup system.
 2. The method of claim 1, wherein the at least one acidis selected from the group consisting of H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃.3. The method of claim 1, wherein the caustic liquid solution comprisesa basic solution for neutralizing the at least one acid.
 4. The methodof claim 1, wherein the caustic liquid solution comprises an NaOHsolution.
 5. The method of claim 1, wherein flowing a caustic liquidsolution downward through the vertical column comprises gravitytrickling or spraying the caustic liquid solution within the verticalcolumn.
 6. The method of claim 1, further comprising flowing spentcaustic liquid solution containing the at least one acid through anopening in the bottom of the column and into a neutralization tank. 7.The method of claim 1, further comprising flowing the treated gas backinto the anaerobic digestion system.
 8. The method of claim 1, whereinthe caustic scrubber is located within the anaerobic digestion systembetween an anaerobic digester and a biogas handling system in order toprotect the biogas handling system and other downstream equipment fromcorrosion.
 9. The method of claim 1, wherein flowing a caustic liquidsolution downward through the vertical column such that the causticliquid solution removes at least one acid from the process gas streamcomprises removing all H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃ from the processgas stream.
 10. The method of claim 9, further comprising preventingacid concentration in all downstream areas despite changing dew pointand condensation.
 11. A method of using a caustic scrubber for thetreatment of process gas from a landfill gas system, comprising:receiving a process gas stream from the landfill gas system; flowing theprocess gas stream up vertically through a vertical column; flowing acaustic liquid solution downward through the vertical column such thatthe caustic liquid solution removes at least one acid from the processgas stream; bubbling out treated gas that is substantially free of acidsthrough an opening at the top of the vertical column; and flowing thetreated gas into a landfill gas cleanup system.
 12. The method of claim11, wherein the at least one acid is selected from the group consistingof H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃.
 13. The method of claim 11, whereinthe caustic liquid solution comprises a basic solution for neutralizingthe at least one acid.
 14. The method of claim 11, wherein the causticliquid solution comprises an NaOH solution.
 15. The method of claim 11,wherein flowing a caustic liquid solution downward through the verticalcolumn comprises gravity trickling or spraying the caustic liquidsolution within the vertical column.
 16. The method of claim 11, furthercomprising flowing spent caustic liquid solution containing the at leastone acid through an opening in the bottom of the column and into aneutralization tank.
 17. The method of claim 11, further comprisingflowing the treated gas back into the landfill gas system.
 18. Themethod of claim 11, wherein the caustic scrubber is located within thelandfill gas system between the landfill gas wells and the landfill gasprocessing and treatment plant in order to protect the landfill gasprocessing and treatment plant and other downstream equipment fromcorrosion.
 19. The method of claim 11, wherein flowing a caustic liquidsolution downward through the vertical column such that the causticliquid solution removes at least one acid from the process gas streamcomprises removing all H₂S, HF, H₂SO₄, H₃PO₄ and HNO₃ from the processgas stream.
 20. The method of claim 19, further comprising preventingacid concentration in all downstream areas despite changing dew pointand condensation.